The present invention relates to a casting pattern having a foamed material body for producing a hollow casting in a metal foundry. The foamed material body consists of a material such as polystyrene particle foam or the like that transforms into a gas in response to the heat of liquid metal such as iron or aluminum. The invention also relates to a process for production of the casting pattern and a process for production of a hollow casting.
Plastic casting patterns consisting of polystyrene particle foam or similar material that transforms into a gas on contact with liquid metal are used in foundries. They are either embedded in binder-free molding material (and possibly subjected to a vacuum before and during casting) or they are molded in bonded molding material. Before insertion into the molds, the foamed material body made of polystyrene particle foam or the like is treated by immersion in, for example, an ordinary foundry blacking consisting of a slurry containing finely ground refractory material. In such a process, the surface of the shell consisting of blacking and facing the foamed material body forms the actual mold while the adjoining molding material such as, in particular, sand, serves only for mechanical support of the blacking shell, which is egg shell-fragile.
One problem with the blacking casting process is the production of the casting pattern itself. Production of a hollow pattern requires four mold shells; that is, two top-half molds and two bottom-half molds. The finished and hardened foamed material body thus consists of at least two shells, the boundary of which is unavoidably outlined on both the inner surface and the outer surface of the finished casting.
If the abutting faces of the shell parts of the foamed material body do not precisely fit together or are not precisely bonded together, blacking can penetrate between the faces. In this respect, problems arise with the use of parts which are difficult to access, particularly those on the inner side of a shell mold. Since the surface of the shell consisting of blacking and facing the foamed material body forms the actual mold of the casting to be produced, blacking projections which penetrate the foam body or seams between the parts thereof cause corresponding indentations in the casting. Such indentations spoil the castings.
A further problem with the blacking casting process is that the foam has a surface which is quite sensitive to pressure or impact forces. Since the abutting edges of foamed material body parts must remain as undamaged as possible, difficulties arise in the handling as well as the storage and transportation of these body parts. If a contact point is imprecisely machined or is damaged in transport or handling, it is generally possible to make repairs by inserting an adhesive piece which, like the foam, gasifies in the heat of the liquid metal. However, this repair process is expensive and often cannot be accomplished with the requisite accuracy, especially at inaccessible points on the inner side of a hollow pattern.